A group of MEMS optical devices is capable of absorbing incident light of a specific wavelength or a wavelength range, such as Infrared light sensors. A MEMS focal-plane-array (FPA) is an image sensing device consisting of an array of MEMS type of optical devices, which are often referred to as image pixels at the focal plane of a lens. MEMS FPAs are used commonly for imaging purposes but can also be used for non-imaging purposes such as spectrometry, lidar, and wavefront sensing.
Some of current MEMS optical devices use combination of heat plates and thermos-sensors. A typical example of an MEMS optical device is illustrated in FIG. 1. This MEMS optical device can be used as a non-cooled Infrared FPA pixel. Referring to FIG. 1, FPA pixel 100 comprises heat plate 102 and thermos-sensor 104. Heat plate 102 is composed of a material or a combination of materials, such as SiOx and SiNx, which have high absorption coefficient (e.g. higher than 60%), wherein the absorption coefficient is defined as “a measure of the rate of decrease in the intensity of incident light of a specific wavelength as the incident light passes through a given material; the fraction of incident light energy absorbed per unit mass or thickness of an absorber. Temperature of heat plate 102 is elevated after being exposed to the incident light of specific wavelength. Change of the temperature is quantitatively measured by thermos-sensor 104. Resistance of thermos-sensor 104 changes, e.g. in a linear way, with the changing of the temperature of heat plate 102.
Because thermos-sensor 104 measures the temperature of heat plate 102, thermos-sensor is often embedded, e.g. in physical contact with heat plate 102 to achieve accurate measurement. It is obviously to know that it is difficult to form a large FPA (e.g. FPA array 106 that is composed of pixel 100 as illustrated in FIG. 2) by using pixel 100 or pixels with a configuration similar to pixel 100, wherein temperature sensing mechanism (e.g. thermo-sensor 102) and light absorbing mechanism (e.g. heat plate 104) are physically contact to achieve accurate measurement. A reason is that when forming in to a large array, a large number of pixels are grouped. During each measurement, detecting current is driven through each thermos-sensor (e.g. thermos-sensor 104) to measure the voltage drop across the thermos-sensor. As a consequence, heat is unavoidably generated by the detecting current, and such unexpected heat dissipate into the heat-plate (e.g. heat-plate 102) and is mixed with the heat generated by the incident light. Measurement errors are caused thereby. Due to the physical configuration that the detecting mechanism (e.g. thermos-sensor 104) is physically embedded into the light absorbing mechanism (heat-plate 102), the error caused by the detecting current through the detecting mechanism (e.g. thermos-sensor 104) is intrinsic and unavoidable. FPAs using pixel of 100 as illustrated in FIG. 2 may not be a large array for the same reason as discussed above.
Another group of current MEMS optical devices capable of detecting incident light for imaging or non-image purposes use the same light absorbing mechanism as pixel 100 in FIG. 1, but different detecting mechanism, such as laser detection. A typical example of such MEMS optical device is illustrated in FIG. 3. Referring to FIG. 3, MEMS optical device 108 comprises deformable membrane 110 and optical antenna 112 that is attached to membrane 110. Antenna 112 can be disposed on the top surface of membrane 110, wherein the top surface is exposed to the incident light. Antenna 112 is capable of absorbing incident light of specific wavelength, such as Infrared light. Antenna 112 is configured according to the desired wavelength or wavelength ranges. In one example, antenna comprises multiple slits, as shown in FIG. 4. Referring to FIG. 4, antenna 112 on membrane 110 comprises a series of parallel elongated slits, such as slit 118. Each slit is composed of a specific material according to the desired wavelength of the incident light. For example, each slit is composed of gold. The geometry of each slit and the slit array are disposed such that the absorption of the desired incident of specific wavelength is maximized, such as higher than 60%.
The absorbed light by antenna 112 converts to heat energy and raises temperature of membrane 110, causing deformation of membrane 110 due to thermos-mechanical effect, as illustrated in dashed lines in FIG. 3. By quantitatively measuring the deformation of membrane 100, density of the incident light can be calculated.
A way to quantitatively measure the deformation of membrane 110 is to use optical interference effect. For example, a beam of laser 114 from laser source is directed to the membrane (e.g. the geometric center of membrane 110). Membrane 110 reflects laser 114; and the reflected laser 116 is collected by a detector. By comparing the reflected laser 116 with a reference laser beam using optical interference, displacement (i.e. deformation) of membrane 110 can be obtained.
The example discussed above with reference to FIG. 3 and FIG. 4 is difficult to form a large array. This is because that if formed into an array each pixel (structure 108 in FIG. 3) of the array is associated with an optical measuring mechanism as discussed above. Deformation of each pixel in the array can then be obtained so as to form an image frame. Providing an optical measuring mechanism for each pixel in FIG. 3 is obviously difficult.
Therefore, what is desired is an optical MEMS device, especially an optical device capable of forming a FPA.